The preparation of ores for further metallurgical processing usually begins with chopping, most often grinding, to the particle size that allows successful flotation ore concentration as the second phase in its preparation. The grinding is done in mills with grinding bodies of different geometries, such as balls, rods, etc. The grinding process causes significant wear of the used grinding bodies and linings of mills which causes the increase in costs not only because of the loss of metal the grinding bodies are made of, but also the cost of transport to the location where the preparation of ore is done. Beside the grinding bodies, the linings of mills, pipelines, cyclones, flotation machines, pumps, etc., are significantly worn, too. For example, the spending of grinding bodies on the location of Veliki Krivelj of the copper mine Bor is between 700 and 800 g of steel per ton of ore.
The ball wear in wet grinding of non-ferrous metal ores is the consequence of both the corrosion and the abrasion processes. The ball wear due to corrosion is many times higher than that due to abrasion.
A significant portion of the costs of ore processing can be attributed to the consumption of grinding media and mill linings. For this reason, experiments directed at lowering steel consumption have both scientific, and practical and economic importance.
In literature, for example Hoey G. R. Can. Mining Met. Bull, vol. 68 No 755 (1975), Balasov G. V., Tjurin N. G., Scerbakov O. K., Cvetnye metally, 11 (1978), Komlev A. M., Scerbakov O. K., Balasov G. V., Cvetnye metally, 5 (1979), it is shown that the consumption of grinding bodies and linings in mills depends on a range of factors, among which the wear of grinding media and linings due to their chemical corrosion has a great influence. Still back in 1937 did Ellis, on the basis of laboratory testing including grinding with balls of different quality, point out the significant influence of corrosion on the wear of grinding bodies. An important part of the corrosion effect in wear of grinding bodies is confirmed by industrial practice. As described by Hoey G. R. Can. Mining Met. Bull, vol. 68 No 755 (1975), at the Wabush plant in Canada, after replacing wet grinding with dry grinding, the ball consumption was reduced from 3.15 to 1.25 kg/t. Thus Sobering and Carlson did conclude that by wet grinding, a small part of grinding media consumption was due to abrasion, while high consumption was the result of the corrosion. F. C. Bond also deems the difference in grinding media consumption between wet and dry grinding can be attributed to corrosion.
As written by Komlev A. M., Scerbakov O. K., Balasov G. V., Cvetnye metally, 5 (1979), experiments aimed at lowering the consumption of grinding bodies had been performed in the Uralmehanobr -institute by slowing down their corrosion rate. Special experiments on a rotating disc electrode indicated that steel consumption in ore pulps was mostly (50–80%) a consequence of electrochemical corrosion as oxydised layers were being permanently removed from metal surfaces.
In the course of grinding, there are certain factors which can lead to the corrosion of grinding media and linings, and they are as follows: the presence of oxygen in the pulp; the presence of oxyde, and particularly sulphide from mineral species which together with the iron metal form electrochemical pairs; chemically aggressive substances; tension in the grinding media, as well as plastic deformation and micro fractures on the surface of grinding bodies which cause differences in potentials.
The pulp pH value in the mill is one of the most important factors influencing the corrosion rate of the grinding bodies and linings. It is common knowledge that the corrosion rate suddenly increases with the decreasing pH value. It has been proved that the high-pressure surface corrodes very quickly. This point is very important for the corrosion of grinding bodies, taking into account that grinding bodies can bear high pressures at the moment of collision. Abrasion in mills also contributes to faster corrosion because oxydised layers of grinding bodies are removed more easily, leaving new and fresh metal surfaces that further corrode intensively.
The mechanism of the effect of corrosion inhibitors has not been properly studied so far. However, for most of them it was determined that they created conditions for a protective film on the metal surface, which could greatly reduce the corrosion rate.
Very efficient corrosion inhibitors in the neutral and base environment are nitrates, chromates, and silicates, as described b&y Scully J. C., The Fundamentals of Corrosion (New York), 1975. All of them have a strong affinity for metal surfaces where they form a thin protection layer which greatly reduces the corrosion rate, which was confirmed by Scully J. C., The Fundamentals of Corrosion (New York), 1975, and Martinko B., Rud. met. zbornik, 1 (1979).
The first experiments regarding the corrosion inhibitor influence on the consumption of grinding bodies in the course of wet grinding were conducted by G. R. Hoey, Can. Mining Met. Bull, vol. 68 No 755 (1975), who achieved very interesting results. Namely, in the laboratory ball mill he carried out wet grinding experiments on copper-nickel ore with the use of various corrosion inhibitors. The results of such experiments show that the use of sodium nitrite, sodium chromate and sodium metasilicate has a great influence on lowering ball consumption in the grinding operation, ranging between 45÷50%.
By examining the influence of sodium nitrite concentration in the liquid pulp phase with pH=12.25, G. R. Hoey has found that the optimum concentration of NaNO2 was within 1.0÷1.5%. He has also found that with the concentration lower than 0.5%, NaNO2 had no influence at all. Optimum sodium chromate concentration with pH pulp value of 8.7÷10.1 was about 0.5%, and optimum concentration of sodium metasilicate with pulp pH value of 12.1÷12.25, was about 1%. The lowest critical concentration below which these inhibitors had no influence at all on the lowering of ball consumption was −0.3% for sodium chromate, and −0.5% for sodium metasilicate.
It should finally be mentioned that G. R. Hoey had conducted all his experiments in a lab porcelain mill, using steel balls as grinding media (0.77% C; 0.8% Mn; 0.06% Cr; 0.12% Ni).
Encouraged by the results obtained by G. R. Hoey which proved that ball consumption in wet grinding could be reduced in certain cases up to 50% by the use of corrosion inhibitors, similar studies were conducted in USSR, as written by Balasov G. V., Tjurin N. G., Scerbakov O. K., Cvetnye metally, 11 (1978). The obtained results are given in brief in Table 1.
TABLE 1The Influence of Certain Corrosion Inhibitors on Ball Consumptionin the Lab MillLiquid phase pulpLoss inConsumptionGround materialcompositionball mass (g)reduction (%)QuartzDistilled water0.736—Sodium nitrite (0.2%)0.56223.6Sodium chromate0.56023.9(0.1%)PyriteDistilled water1.360—Sodium hydroxidepH = 13.180.57757.6Copper-Zinc OreDistilled water1.110—Sodium nitrite (1.1%)0.59246.7
The results shown in Table 1 reflect not only the inhibitor influence, but also the mineral content and pulp pH value on the ball consumption in wet grinding.
The use of depressants in the preparation of suliphide ores of non-ferrous metals is very common, for which cyanides, zincsulphate, sodiumsulphate, etc. are most often used as depressants. Polymetallic ores lead-zinc are the most significant source for getting these two metals. Certain natural resources have caused the ores of lead and zinc to be observed as a united ore apart from its polymetallic composition, i.e. the lead and zinc content as their economic value. Metallurgic processing of this ore sets certain conditions in terms of quality of the lead and zinc concentrates, where those concentrates are obtained in the phase of the preparation of ore for the metallurgical processing. The technical problem appearing in the preparation of these ores is the process of separating and obtaining two quality concentrates: lead and, zinc. It is customary that the collecting of ores from the flotation pulp is done by using xanthates that are very efficient at sulphide ores, if prepared in the base medium, with pH value between 7 and 9.
The fact is that today the collection of galena in the lead-zinc ore, in industrial production, is done by using a depressant for sphalerite, by what it is achieved that sphalerite, pyrite, and other sulphide materials not to be the constituent part of the galena concentrate. The most important and industrially most applied depressants practically from 1922 have been the cyanides, i.e. NaCN. Beside it, ZnSO4 has been used, too, being introduced for the first time in the Sheridan-Griesvold process. Apart from these, there are other depressants, but they have not managed to eliminate the cyanides from this use because cyanides give better effect. However, since cyanides are particularly poisonous, their use is undesirable, but up to now it could not have been avoided from economic reasons. Although after being used they are collected at the bottom of a dump, there is a constant threat that they might, by diffusion through soil, get into water flows and pour out of the dump if there is damage on the barrier of the dump, what has recently happened in a damp in Romania when the river Tisa was polluted.
When the question is about the sulphide copper ores, in the preparation of the ore by flotation, xanthates, dithiophosphates, mercaptanes, thiourea, etc., are used as collectors, and all of them show good effect in flotation. However, the problem while using those collectors is that with-useful copper minerals, such as halcozym, chalcopyrite, borite, bornite and cubamite, at the same time they collect the pyrite, too, which makes the metallurgic processing of the concentrate significantly more difficult because of the increase in sulphur concentration.
Concentrating ores by lead-zinc flotation is practically done by two technological processes, which are the selective flotation of useful materials or the collective flotation of useful minerals. The process of collective flotation of lead and zinc minerals from polymetallic ores is rarely applied, and only when certain kinds of collective concentrate could be metallurgically processed later. The best known of those processes is the process known as “Imperial Smelting”.
In most lead-zinc ores the process of selective flotation is applied. In that process the depressant is added in order to tip the sphalerite and the collector for collecting galena, and then the tipped sphalerite is activated by adding coppersulphate and collected by the appropriate collector. Most often used depressant for sphalerite is cyanide, and as collectors of sulphide lead and zinc minerals the xanthates, dithiophosphates, thiourea and mercaptanes are used most often.
At deposits of non-ferrous metal ores beside sulphide minerals, oxyde minerals appear, too, for example, azurite (copper oxydesulphate) malachite (copper oxyde carbonate), then in lead-zinc ores as. ZnSO4, etc.
When the copper ore is in question, there is no doubt that its sulphide minerals are of the greatest economic importance and it is supposed that more than 85% of the copper production in the world originates from its sulphide ores. However, oxyde ores, too, have, and can have, a significant economic effect, or, more precisely, oxyde copper minerals, like malachite, azurite, cuprite, chryocol, brochantite, chalacnite, and other water-soluble minerals. Oxyde copper minerals flotate not as well as sulphides. The tests have proved that in one single mineral several chemical bonds are present—ionic, covalent and metallic. With the increase of the contribution of ionic bonds in a mineral, the mineral surface reacts more actively with water bipoles, so more stable and thicker layers of water are formed on the mineral surface, which makes the hydrophobisation of the mineral surface more difficult by the collector. The reason for this bad effect of the existing collectors in oxyde mineral flotation is explained by strong activity of water bipoles because of the presence of oxygen, which has great thickness and consistency of hydrate layers on mineral surfaces as a consequence. Since collector anions have large dimensions, they defund with difficulty through the thick and consistent hydrate layers, so the hydrofobisation process is made considerably more difficult. The bond between the collector anions and cations of the crystal grid f the oxyde mineral is very weak, so it is often the case that even the bonded collector is removed easily from metal surface, which altogether decreases the effect of the collector in the flotation phase. That is the reason why, for the sake of a successful flotation of oxyde copper minerals with the help of sulphide collectors, the precious partly sulphidisadion of the minerals surface is done leading to the surface compounds of sulphido-sulphate type. That additional phase which increases the overall costs is mostly done by the application of sodium sulphides, although K2S, BaS and H2S are used, too. The sulphidisation result is that copper sulphide membrane improves hydrofobisation of oxyde mineral surface and facilitates the reaction of the collector with sulphidised mineral.
In order to make a difference between the up-to-now used reagents for the preparation of non-ferrous metals from the reagent according to this invention, it is important to say that in the methods of ore preparation so far, the corrosion inhibitor, if used, is added to the mills in the wet grinding phase, and depressants, collectors, foamers and other reagents to the flotation machines the flotation process is done in.